1. Field of Invention
The invention relates to hydrogen storage systems, more particularly to the storage of hydrogen in systems that include nanostructures of combinations of light elements.
2. Description of Related Art
Hydrogen storage is the key unsolved problem of producing fuel cells for hydrogen-powered automobiles or portable energy devices. In particular, storing hydrogen in large quantities safely and in a light container has proved prohibitively difficult so far.
Several different techniques have been developed to tackle this problem. In some approaches hydrogen is stored in tanks under high pressure, for example, 300 atm. In other techniques hydrogen is liquefied at temperatures below 20 K with a helium-based cooling system. Both of these techniques pose problems for practical use in automobiles. All of the hydrogen is available for catastrophic release in an accident, raising the risk of explosion or fire. Furthermore, in order to store enough hydrogen to match the range of present day automobiles, the container has to have a volume of at least 50 gallons. Also, both in the high pressure technique and in the helium cooled technique the required containers are heavy, and therefore inefficient for storage. Finally, both techniques also consume a lot of energy for generating the high pressure or for liquefying the hydrogen.
Some other techniques adsorb hydrogen into solid materials. Several types of materials have been studied in this respect, including metal hydrides and glass microspheres. However, the materials investigated so far all have low hydrogen storage capacity, making them non-competitive with gasoline.
Hydrogen can be stored in carbon nanostructures, such as graphite and carbon nanofibers, according to the papers of A. Dillon et al. in Nature, vol. 386, p. 377 (1997), A. Chambers et al. in J. Phys. Chem. B vol. 102, p. 3378 (1998), and E. Poirier et al. in Int. J. of Hydrogen Energy, vol. 26, p. 831 (2001), and according to U.S. Pat. No. 5,653,951: xe2x80x9cStorage of hydrogen in layered nanostructures,xe2x80x9d by N. Rodriguez and R. Baker; and U.S. Pat. No. 4,960,450: xe2x80x9cSelection and preparation of activated carbon for fuel gas storage,xe2x80x9d by J. Schwarz et al. Furthermore, hydrogen storage in Al and Si containing zeolites and microporous materials has been explored previously.
Nanostructures can be defined as atomic structures that have a spatial extent of less than a few hundred nanometers in one, two, or all three dimensions. A class of nanostructures is formed by planar networks, sometimes referred to as layered compounds. Layered compounds are often formed by elements coupled with sp2 bonds. The origin of the sp2 bonds will be presented on the example of elements of the second row of the periodic table, including boron, carbon and nitrogen.
FIG. 1 shows an example of a second row element 4 coupled with sp2 bonds, or orbitals, 8 to three other elements 12. The s orbital of the second row elements is filled with two electrons, and the p orbitals are partially filled. For example, boron has one electron, carbon has two, and nitrogen has three electrons in the p orbitals. When the second row elements form chemical bonds, one of the s electrons is promoted into an empty p orbitalxe2x80x94for example into the pz orbital in carbon, leaving only one s electron. This one s electron and two of the p electrons first hybridize into three s hybrid orbitals. The remaining p electronsxe2x80x94none in boron, one in carbon, and two in nitrogenxe2x80x94occupy an orbit that does not participate in the bonding. The three hybridized electrons repel each other, and hence form three sp2 orbitals 8 as far as possible away from each other. An optimal configuration is when the three sp2 orbitals 8 make 120 degrees with each other, defining a plane. Connecting several second row elements with planar sp2 orbitals 8 spans the defined plane, thus forming the aforementioned planar networks.
Possible planar networks of the sp2 bonded materials include triangular lattices. Large sections of a planar network can be deformed to create various nanostructures. Nanostructures that are based on sp2 bonded triangular lattices include different classes of nanotubes, nanococoons, nanoropes, nanofibers, nanowires, nanohorns, and nanocages.
Storing hydrogen in sp2 bonded nanostructures has the following advantages. Hydrogen, adsorbed to the nanostructures, desorbs over a range of temperatures, and thus it is not available for catastrophic release, for example, in case of an automobile accident. Furthermore, because of their large surface area, nanostructures are capable of adsorbing very large quantities of hydrogen, giving rise to a much higher weight % storage efficiency than the aforementioned high pressure and cooling techniques.
However, the above works have the following disadvantages. Typically they considered hydrogen storage at ambient temperatures, where the storage capacity fell far short of the theoretical value, making those works economically non-viable. Also, the works that considered storage at other temperatures reported insufficient storage efficiencies.
In particular, U.S. Pat. No. 5,653,951 considered hydrogen storage in carbon nanostructures, utilizing chemisorption. As described below in detail, chemisorption binds hydrogen to the carbon nanostructure by forming a chemical bond that is typically quite strong. Therefore, chemisorptive bonds can change the chemical composition and structure of the storage material itself. This is a drawback for storage applications, as the storage system has to be operated cyclically without structural degradation in order to be useful.
Also, because of the formation of chemical bonds, the hydrogen might be recovered from the storage material in an altered chemical form, for example, methane. This again reduces the usefulness of storage materials, which form chemisorptive bonds.
Therefore, there is a need for hydrogen storage systems that contain sp2 bonded nanostructures, wherein the chemical composition of the nanostructure is selected to ensure high storage efficiency, the storage system operates at technically advantageous temperatures, and in particular wherein the mechanism of hydrogen adsorption is not chemisorption.
In accordance with the invention, a hydrogen containing nanostructured storage material is provided, where the hydrogen is adsorbed to the nanostructured storage material by physisorption. The nanostructured storage material includes light elements, belonging to the second and third rows of the periodic table. More specifically, the light elements are selected from Be, B, C, N, O, F, Mg, P, S, and Cl. The chemical composition of the nanostructured storage material is such that the desorption temperature, at which hydrogen desorbs from the nanostructured storage material, is greater than the liquefaction temperature of nitrogen, 77 K. Some chemical compositions that give rise to a desorption temperature in excess of 77 K are: BxCyNz, BN, BC2N, MgB2, Be3N2, BeB2, B2O, B, BeO, AlCl3, Al4C3, AlF3,Al2O3, Al2S3, Mg2Si, Mg3N2, LixNy, LixSy, and NaxSy, where x, y, and z are integers.
The nanostructured storage material is formed as a layered network of light elements, coupled with covalent sp2 bonds. The layered network can be a triangular lattice, a nanofiber, a nanoplatelet, a single walled nanotube, a multi walled nanotube, a nanocage, a nanococoon, a nanorope, a nanotorus, a nanocoil, a nanorod, a nanowire, and a fullerene. The layered network can also have a heterogeneous form, including a combination of the above structures, as well as embodiments where various parts of the network can have different chemical composition.
According to another embodiment of the invention, a hydrogen storage system is provided. The hydrogen storage system includes a container and a nanostructured storage material within the container, wherein the nanostructured storage material includes light elements, and the nanostructured storage material is capable of adsorbing hydrogen by physisorption. The nanostructured storage material can be, for example, any of the above-described embodiments. The nanostructured storage material can be combined with a hydrogen distribution system to facilitate the efficient flow of hydrogen.
In some embodiments the hydrogen storage system further includes a cooling system, capable of cooling the nanostructured storage material below the desorption temperature of hydrogen in relation to the nanostructured storage material. In some embodiments the cooling system includes a middle container within the container, separated by vacuum, an inner container within the middle container, and liquid nitrogen between the middle container and the inner container. The nanostructured storage material is within the inner container. Some embodiments contain a heater to control the temperature of the nanostructured storage material.